PPPChapter9Exercise

Part 2: Unit 9

You have been tasked with "reusing" what's already there. You've already pulled out one or more components to support that functionality -- catalog maintenance -- how does your component measure up?

We just discussed several component coupling principles:

• Acyclic Dependencies Principle
• Stable Dependencies Principle
• Stable Abstractions Principle

We also just learned about several metrics we can use to evaluate our components:
Instability

• $\[I = \frac{C_E}{(C_E + C_A)}$
  
  Where:
  ||
  $C_E$ || Efferent Coupling --> Fan Out (E in Efferent --> Exiting) ||
  
  ||
  $C_A$ || Afferent Coupling --> Fan In (A in Afferent --> Arriving) ||

Abstractness

• $A = \frac{N_a}{N}$
  
  Where:
  ||
  $N_a$ || Number of abstract classes in component ||
  
  ||
  $N$ || Number of classes in component ||

Distance from Main Sequence

• $D = | A + I - 1 |$
  
  Where:
  ||
  $A$ || Abstractness ||
  
  ||
  $I$ || Instability ||

---

What are the values for the original diagram?
Middle Tier
$C_E = 0$
$C_A = 7$
$I = \frac{0}{0+7} = 0$
$A = \frac{2}{14} = 0.143$
$D = |0.143 + 0 - 1| = 0.857$

UI Tier
$C_E = 7$
$C_A = 0$
$I = \frac{7}{7+0} = 1$
$A = \frac{1}{5} = 0.2$
$D = |1 + .2 - 1| = 0.2$

---

What to do next?

• Now perform these calculations for your new component configuration. How do you compare?
• What can you say about the components that have a "large" distance from the main sequence?
• Knowing what you now know, work through a second round of componentization.

---

Final Observations

• What is your experience with doing this kind of work?
• What tool support have you used or do you see available to assist in this work?
• Why go to all this trouble?